A humidity sensor, currently used in various fields, is used in a microwave oven which heats and cooks foods by utilizing the principle of dielectric heating, to increase the cooking efficiency. Automatic cooking may be enhanced by monitoring the humidity in the cooking chamber using a humidity sensor. Conventional art using such a humidity sensor is disclosed in Japanese Patent Laid-Open No. 181288 (1983). When a comparator compares a voltage signal from the humidity sensor with a reference voltage which is produced by a microprocessor and converted through a reference voltage generator, the resolution of the reference voltage or the response speed can be varied by increasing and/or decreasing the number of bits from the microprocessor according to kinds of food to be cooked. The conventional apparatus applies constant DC voltage to the branch of a humidity sensor and a fixed resistor connected in series, and then determines whether to complete the cooking process by comparing the voltage at the junction of the humidity sensor and the fixed resistor with the reference voltage. The humidity sensor is operated with a constant DC voltage so that an additional rectifier circuit is not needed and the interface with peripheral digital circuits is simplified. However, when constant DC voltage is applied to the humidity sensor for a long time, the sensing efficiency of the humidity sensor decreases.